New York Times columnist and author of the new e-book, The First 20 Minutes Personal Trainer, Gretchen Reynolds gives us the final world on how to make your exercise count once and for all.

Don't focus on strengthening your core

Don't focus on strengthening your core

Well, it looks like that hundredth sit-up was probably a waste. When scientists tested athletes, such as football players, who would seemingly need super-strong cores to stay upright on the field, they found no correlation between core strength and performance. That's because the real hallmark of success is actually core stability. "An unstable core throws off all the muscles that circle your back," says Reynolds. "The primary purpose of the core muscles should be to hold your spine in place." To raise core stability, don't do sit-ups, which are usually performed incorrectly, and can lead to serious lower-back injuries. Instead, try three exercises: 10 proper crunches, which require laying flat on the ground with your knees up and your hands under your lower-back, lifting your head and neck until they're only a few inches off the ground; a series of side planks; and 10 bird-dogs, for which you get on all fours, and lift and extend your opposite arm and leg, hold it for a few moments, and switch sides.

People respond to exercise differently

People respond to exercise differently

Ever wondered how your weightlifting buddy has gotten stronger so quickly? "What's becoming clear is different people with different genetic make-ups will respond differently to the exact same workout routines in terms of how fit they get, how much weight they lose, and how much they enjoy it," says Reynolds. The only way to determine the best program for you is through trial and error, and even variations of cardio — like swimming, running, and cycling — can make a difference. "The human body is designed for movement, but there are lots of genetic variations in terms of what movement would be good for you," Reynolds adds.